The invention relates to a coaxial transmitting and receiving optical system for an electro-optical range finder. The range finder is of the type which transmits to and receives from a remote reflector a modulated beam of light for determination of the distance. The system also has an integrated sighting telescope for sighting the reflector or some other target.
Special theodolites called tacheometers are known which permit the accurate measure simultaneously of directions and distances. The direction is normally measured by reading off graduated circles. The distance is measured by means of a reticule scale in the telescope and a test plate set up at the target point. For some time, electro-optical range finders have been used for distance measurement purposes. They are based on the principle of phase measurement or transit time measurement and comprise a transmitter with a transmitting optical system and a receiver with a receiving optical system. It is known to combine such devices with a theodolite. This known combination, then, comprises the transmitting and receiving optical system of a range finder together with a sighting telescope. For reasons of reduced complexity, reduced weight, and the simplification of the mutual adjustment of the various optical systems, it is advantageous to construct the transmitting and receiving optical system as a coaxial system with the sighting telescope integrated therein.
In a coaxial system of the above-described type, at least one beam splitting process is necessary in order to separate the transmitting beam path, receiving beam path, and the beam path of the sighting telescope. This may be accomplished in a known manner by the use of either a planar plate inclined relative to the optical axis and with a selectively reflecting dielectric coating system which on one side acts against air or by a photometer cube with a selectively reflecting, dielectrically mirrored photometric surface.
However, a coaxial system with an inclined planar plate or a photometer cube has disadvantages. As is known, in a non-parallel beam path, such as in the beam path of a sighting telescope, an inclined planar plate produces astigmatic errors which can only be compensated optically at high cost. In the case of the photometer cube on the other hand, technically unavoidable and spectrally-dependent variations of the reflection factor over the dielectrically mirrored photometric surface causes, for instance in the case of temperature variations, a change in the spectral characteristics of the radiation in the transmitting and receiving beam path. This leads to a deterioration of data transmission and evaluation and consequently to a reduction in the precision of measurement. In the case of an electro-optical range finder of a specific type, the permitted spectral variation of the reflection factor of the selectively reflecting beam splitting surface must typically be less than 1% for a distance measurement precision of .+-.5 mm. However, in the case of dielectric coating systems which act on glass on both sides, such as in a photometer cube or those systems designed for an angle of incidence of approximately 45.degree., this value cannot at present be attained.